Global warming is one of the major causes of coral reef bleaching. Abnormally high sea temperatures kill the tiny marine algae that are essential to coral health. In Australia, a recent aerial survey has shown that 95% of the northern Great Barrier Reef is bleached - far worse than previously thought. Of the 520 reefs surveyed only four showed no evidence of bleaching. It is estimated that 60% of the world's coral reefs will be lost by 2030.